The Moscow theater hostage crisis (also known as the 2002 Nord-Ost siege) was the seizure of the crowded Dubrovka Theater by 40 to 50 armed Chechen terrorists on 23 October 2002, which involved 850 hostages and ended with the death of at least 170 people. The attackers, led by Movsar Barayev, claimed allegiance to the Islamist separatist movement in Chechnya. They demanded the withdrawal of Russian forces from Chechnya and an end to the Second Chechen War.

Due to the layout of the theater, special forces would have had to fight through 30 metres (100 ft) of corridor and advance up a well-defended staircase before they could reach the hall in which the hostages were held. The attackers had numerous explosives, with the most powerful in the center of the auditorium. Spetsnaz operators from Federal Security Service (FSB) Alpha and Vympel, supported by a Russian Ministry of Internal Affairs (MVD) SOBR unit, pumped an undisclosed chemical agent into the building's ventilation system and began the rescue operation.All 40 of the insurgents were killed, and up to 130 hostages died during the siege, including 9 foreigners, due to the toxic substance pumped into the theater. The identity of the gas was not disclosed at the time, although it was believed by some to have been a fentanyl derivative, such as carfentanil. A study published in 2012 concluded that it had been a mixture of carfentanil and remifentanyl.The same study pointed out that in a 2011 case at the European Court of Human Rights, the Russian government stated that the aerosol used was a mixture of a fentanyl derivative and a chemical compound with a narcotic action.